fsck stands for "file system check" and it is used to check and optionally repair one or more Linux file systems. Normally, the fsck program will try to handle filesystems on different physical disk drives in parallel to reduce the total amount of time needed to check all of the filesystems (see: man fsck).

The Arch Linux boot process conveniently takes care of the fsck procedure for you and will check all relevant partitions on your drive(s) automatically on every boot. Hence, there is usually no need to resort to the command-line unless necessary.

1 = First file system (partition) to check; / (root partition) should be set to 1.

2 = All other filesystems to be checked.

Troubleshooting

Can't run fsck on a separate /usr partition

Make sure you have the required hooks in /etc/mkinitcpio.conf and that you remembered to re-generate your initramfs image after editing this file.

Make sure that the bootloader has ro on the "APPEND" line in /boot/syslinux/syslinux.cfg (for Syslinux). GRUB doesn't need one; it is added automatically when you generate a .cfg. For an explanation as to why you need "ro", see this post.

Check your fstab! Only the root partition needs "1" at the end, everything else should have either "2" or "0". Carefully inspect it for other typos, as well.